A rod-in-tube method is one of typical methods for producing a glass preform for use in the fabrication of an optical fiber and comprises steps of inserting a glass rod constituting a core in a glass tube constituting an cladding having a lower refractive index than the core and heating and fusing the tube and the rod to collapse a gap between them and to integrate them. The glass preform produced by the rod-in-tube method has, however, several drawbacks such that an interface between the core and the cladding tends to have many defects such as bubbles and impurities so that the optical fiber fabricated from the glass preform has large attenuation of light transmission.
To overcome the above drawback of the rod-in-tube method, it was proposed to supply and oxidize a gaseous mixture of a metal halide and oxygen so as to deposit the decomposed product on the outer surface of the rod and the inner surface of the tube (cf. Japanese Patent Publication No. 46898/1984).
It was also proposed to supply a glass-forming raw material containing at least one of fluorine or boron together with oxygen gas in the gap between the rod and tube and heating them to form a quartz glass layer having a lower refractive index than that of the glass rod on the surface of the rod and the inner surface of the glass tube (cf. Japanese Patent Kokai Publication (unexamined) No. 135810/1979).
In addition, it was proposed to inject a gaseous treating agent for modifying the glass surface in the gap between the tube and the rod before fusing them and collapsing the gap between them, which agent does not contain any material having a higher boiling or sublimating temperature than the fusing temperature of the glass (e.g. silicon halide) and can be evaporated before substantially complete fusing of the rod and tube (cf. Japanese Patent Publication Nos. 6261/1984 and 52935/1983).
However, it has been found that when a single mode optical fiber with large capacity to be used for telecommunication at a wavelength range of 1.3 to 1.6 .mu.m is fabricated from the glass preform produced by each one of the methods disclosed in said Japanese Patent Publications, it is impossible to fabricate any optical fiber having attenuation of light transmission of less than 0.5 dB/km at a wavelength of longer than 1.3 .mu.m since absorption loss due to the hydroxyl groups and scattering loss due to interfacial defects between the core and the cladding are large.
As the result of the extensive study by the present inventors, it has been found that sources of the hydroxyl groups are water which is diffused and chemically adsorbed on the surfaces of the glass tube and rod and moisture contained in an atmosphere between the tube and the rod, so that the interfacial area between the cladding and the core of the optical fiber contains the hydroxyl group in a high concentration.
Particularly, according to the methods disclosed in Japanese Patent Publication Nos. 6261/1984 and 52953/1983, when the rod-tube composite is thermally treated before collapsing the gap in the presence of the treating agent including a fluorine-containing compound (e.g. SF.sub.6 and CCl.sub.2 F.sub.2), the hydroxyl groups chemically adsorbed on the surface of the rod and the inner surface of the tube are removed by the etching effect of the treating agent. However, when the composite is heated at a temperature lower than 1900.degree. C. at which the rod is not deformed, the surface of the rod and the inner surface of the tube are roughened and the glass becomes opaque like ground glass. If such the surface roughened rod is fused together with the tube to form a transparent glass preform, the optical fiber fabrication from the preform has structural defects at the interface between the cladding and the core, which results in increase of attenuation of light transmission. On the contrary, when the rod-tube composite is heated before fusing it in the presence of a treating agent not containing fluorine (e.g. SOCl.sub.2 and CCl.sub.4), the hydroxyl groups still remain in a high concentration at the interface between the cladding and the core of the fabricated optical fiber since such treating agent has no etching effect, this resulting in absorption loss due to the hydroxyl groups.
When the glass layer is formed on the surface of the rod and the inner surface of the tube according to the methods disclosed in Japanese Patent Publication No. 46898/1984 and Japanese Patent Kokai Publication No. 135810/1979, the interface between the cladding and the core of the fabricated optical fiber still contains the hydroxyl groups at a high concentration, when again causes large absorption loss. When the rod-tube composite is integrated with introducing only nitrogen, oxygen or helium in the gap between the rod and the tube, it is difficult to keep a dew point (namely moisture content) constant due to leak in a pipe line and thereby it is difficult to economically fabricate, with good reproducibility, the optical fiber containing the hydroxyl groups in a low concentration and having low attenuation of light transmission at a longer wavelength range